Attempts to generate HAV vaccines have been unsuccessful to date due to several factors including poor growth of the virus in tissue culture, absence of in vitro markers of attenuation, and the fact that isolated viral proteins do not display virus neutralizing epitopes. As an alternative to classical killed virus or a attenuated virus vaccine approaches or recombinant DNA derived subunit vaccines, a defective HAV approach will be developed and tested as a HAV vaccine. A new HAV strain that causes lytic infection and grows to high titer in tissue culture will be used. In Phase I, the genome of this fast growing HAV variant will be cloned and the biological activity of the isolated clone will be assessed by transfection into susceptible human hepatoma cells. In Phase II, deletion or substitution mutations will be introduced into the 2C region of the genome to generate a defective HAV. A complementing cell line expressing the 2C gene will be constructed and used to grow the defective virus. Once the defective virus and complementing cell line are constructed, studies to assess the stability of the mutant virus and the production level of the virus will be done in order to determine the feasibility of using the virus as a vaccine.